1. Field of the Invention
The present invention pertains to a tandem propeller assembly for use on a marine propulsion unit and, more particularly, a tandem propeller assembly which utilizes a single shock-absorber arrangement between a drive shaft of the marine propulsion unit and both of the tandem propeller units.
2. Discussion of the Prior Art
U.S. Pat. No. 2,672,115 discloses the concept of utilizing tandem propellers in a marine propulsion unit. In this patented arrangement, each of the propellers are attached to a drive shaft of the marine propulsion unit. By appropriately designing their diameters, pitch and relative positions, the tandem propellers can be used to improve the propulsion efficiency with respect to load in various types of watercrafts.
In this prior art arrangement, the tandem propellers are not affixed to the drive shaft by a shock-absorber. Instead, a complex protective structure is utilized to protect the propulsion system from being damaged when drifting wood or other debris strikes either of the propellers. Although it is technologically possible to construct a specialized shock-absorber for each of the propellers when a plurality of propellers are mounted in tandem, this would inherently lengthen the collars associated with the propellers. The longer the propeller collar, the greater the contact surface with the water which results in increased propulsion resistance and speed loss.
Therefore, there exists a need in the art for a marine propulsion unit which utilizes a tandem propeller assembly incorporating a shock-absorber between the propellers and the drive shaft. In addition, there exists a need in the art for a tandem propeller assembly incorporating a shock-absorber wherein the length of the collars associated with the propellers can be minimized so as to maintain a low propulsion resistance during operation. Finally, there exists a need in the art for a shock-absorbing arrangement for use with a tandem propeller assembly of a marine propulsion unit which is simple in structure and which can functionally protect the propulsion system from damage when the propellers are obstructed by drifting wood or other debris during operation.